1. Field of the Invention
The present invention relates generally to optical switches. In particular, the invention relates to bistable optical switches that can be implemented in an integrated array.
2. Description of the Related Art
Fiber optics are used in a wide variety of applications. One application is in the area of communication systems. In such application, information signals, in the form of modulations of laser-produced light, are transmitted to, and received from, a plurality of terminals through fiber optic cables. A switching network is provided to selectively direct the information signals to, and from, the terminals. Typically, the switching network converts light emitted by transmitting one of the terminals into corresponding electrical signals. The electrical signals are directed to a selected output port of the switching network by electrical circuitry in the switching network. The electrical signals at the output port are converted into corresponding modulated light for retransmission through fiber optic cables to a receiving one of the terminals. The conversion of the modulated light to electrical signals and then back into modulated light energy, together with the electrical switching circuitry, requires the use of expensive components and/or restricts the potential bandwidth of the data communication system.
Recently, a number of optical switching systems have been developed. As the name implies, in optical switching networks, light emitted by typically any one of the cables is switched to another one of the cables without requiring intermediate conversion of the transmitted light data into electrical signals and then re-conversion of the electrical signal into light data. U.S. Pat. No. 5,255,332, Welch, et al., issued Oct. 19, 1993, entitled "NXN Optical Crossbar Switch Matrix", (hereinafter "Welch"), describes one such optical switching system and provides a survey of optical switches that employ various schemes to achieve the switching.
One optical switch described in Welch is that of Jackel, et al., U.S. Pat. No. 4,988,157, issued Jan. 29, 1991, entitled "Optical Switch Using Bubbles", (hereinafter "Jackel"). The Jackel optical switch is particularly useful as a bistable cross-connect matrix. Parallel input waveguides and parallel output waveguides are formed on a substrate at perpendicular angles so as to intersect. A forty-five degree slot is formed across each intersection and is filled with a fluid having a refractive index matching the waveguide material. Electrodes are positioned adjacent the slots and are selectively activated to electrolytically convert the fluid to gaseous bubbles. The bubbles destroy the index matching to the waveguide material and cause light to be reflected by the slot rather than propagate across the slot. In the presence of a catalyst, a pulse of opposite polarity, or of sufficient size and of the same polarity, will destroy the bubble.
Thus, the electrolyte associated with each intersection is selectively activated to alternately form and remove a gaseous bubble in the index-matching fluid at the slot. When the index-matching liquid fills the respective slot, the optical signal simply passes through the intersection. When the gaseous bubble displaces the index-matching liquid however, the index mismatch at the waveguide-slot interface causes the light to be reflected from the side of the slot into the intersecting waveguide. The bubble can be made bistable. That is, once created, the bubble will remain for periods of days.
Although bistable, the switch will ultimately fail when the bubble dissipates. Therefore, the switch must be periodically tested and/or reset to ensure proper operation during long-term periods (i.e., a week or longer). Furthermore, if a liquid that has an index of refraction closely matching that of the waveguides, such as an oil, is employed in the switch, the liquid can crack and oxidize. Thus, over time, the liquid will degrade into a varnish and the switch will not operate properly. On the other hand, water or light alcohols will not decompose when used in the switch. These liquids, however, do not have an index of refraction that closely matches that of the waveguide, thereby introducing reflection-associated crosstalk into the switch. Moreover, using electrolysis to create and remove bubbles can result in an unacceptably slow switching time for some applications.